Aluminum-alloyed corrosion-resistant metal powders and related products and processes



Patented Oct. 27, 1953 ANT METAL POWDER/g AND RELATED PRODUCTS ANDpeoces'sts George Stern, Mamaroneclq, Seymour J..; Sindeband,Chappaqua,. and Joseph l Seaplan, Xonkers, N; Y., assignors to AmericanElectro Metal corporation, Yonkers, N. Y;, a corners tiofi of MarylandNo Drawing. Application March 31-, 1950, Serial No. 153,289

(or. fi -t3) '10 Claims.

This inventionrelates to readily compactible and moldable powders whichresist corrosion, shaped bodies of such powders, and methods forproducing the same, some aspects of the invention being of a broaderscope.

Among the objects of the invention are POW ders of metal particles whichare in a readily deformably plastic condition and maybe com; pacted intorelatively strong metal bodies with relatively low pressure, at leastthe surface layer of the metal powder particles having alloyed therewithaluminum from a gaseous or liquid compound at an elevated temperature;

A particular object of the invention is such readily compactiblemoldable powder containing essentially powder particles of atleast onemetal of the group consisting of iron, nickel and cobalt, and ofmixtures and alloys of said metals, which particles are in a rcadilydeformable, relatively soft plastic condition, at least the surfacelayer of said particles having alloyed therewith aluminum which has beendeposited thereon at an elevated temperature from an aluminum com-Another object of the invention is such alumimim surface=alloyed softmoldable metal powder combined with similar surface alloyed powderparticles of at least one metal of the group consisting of moly' bdermm,tungsten, chromium, vanadium, manganese; or mixtures or alloys thereof.

Further objects of the"- invention include com-- pasted bodies made ofsuch surface-alloyed metal powder particles, methods for the econom=ical production of such surface alloyed' soft mold= able metal powderparticles, and economical production' or compacted bodies from suchpowder particles of the invention.

The foregoing and other objects of the invention will be best understoodfrom the following description of specific exemplifications thereof.

For'ye'ars past various corrosion=resistant prod ucts such as gears,valve parts; bearings, filters, and the like, have been made ofstainless steel powders by powder metallurgyt'echniques. The stainlesssteel powders generally used for such bodies have been obtained frompreviously propared stainless" steel ingots, for instance, by subjecti-ng a stainless steel ingot to grain boundary corrosion followed bydisintegration, or by melting astainless steel ingot and atomiz'ing itin themol= ten state. The SO-Pffidflijd particles of such stainlesssteel powders are of high density and hardness; and require high-moldingpressure of at least- 40 to 45 t. i.- (tons per square" inch) which 2 ismuch too high for economical production pr hiolded products, and resultsin rapid wear and frequent break-down of the molding dies. Asa result,the production of molded products from stainless steel powders ha foundonly limited use.

What has been said above about stainless steel powder, which is an alloyof iron, nickel and chromium, applies also to various other generallys'i i nilar cg'mmmercially available alloys, such as alloys of iron,cobalt and nickel with aluminum.

According to the invention the foregoing diflicultics heretoforeencountered in the production of corrosion-resistant ferrous metalbodies by powder metallurgy techniques are overcome by the provision ofareadily compactible and moldable metal powder of relatively soft ferrouspowder particles, at least the surface layer of the powder particleshaving combined therewith aluwhich has been deposited thereon at anelevated temperatu'reso as to render the powder particlescorrosion-resistant, the surface alloyed metal powder particles beingsufiic iently soft and plastically deformable as to make it possible tocompact them with relatively low pressure, for instance, only 25 to 30t. s, i. into bodies of greater strength than molded bodies made bycompactin prior art powder of generally similar composition.

v yarious commercially available soft, plasticall-y readily deformableiron powders are suitable for producing corrosion-resistant softmoldable metalpowders of the invention.- Among such available andsuitable soft iron powders is sponge iron powder having a carbon contentof .l to 2% (unless otherwise specifically stated,-

all proportions are given herein by weight), elec- .trolyticallyproduced iron powder, carbonyl iron powder, eddy mill iron powder, andatomized molten iron powder of similar iron content. The followingimpurities present in such available powders,- to Wit, .3% Mo; .-2% Si,and other minor impurities usually present therein, do not impair theirusefulness for practicing the invention.

In addition, many relatively hard and'difiicult to compact fine ironpowders will, in general, when subjected to the alloying process of theinvention herein described, become softened and acquire characteristicswhich render them readily moldable.

In accordance with a phase of the invention based on the originalconcept of producing a1umin -rm surface-alloyed ferrous ironpowder-"relatively soft powder particles of difiefefit met= ale; to wit; one ormore metals of the group eluding iron, nickel and cobalt, are combinedin proper proportion, and a desired proportion of aluminum from analuminum compound is deposited on and difiused into the interior of suchmetal powder particles so as to yield a soft powder body of soft powderparticles combining aluminum with the other metal or metals inproportions required for producing alloyed bodies of desiredcharacteristics and containing aluminum and such metals in the desiredproportions.

Any of the known methods for depositing aluminum from an aluminumcompound on the surface of a metal body heated to an elevatedtemperature may be used for depositing on and diffusing aluminum intometallic powder particles in accordance with the principles of theinvention.

Among such known aluminum alloying processes, one that was foundparticularly suitable for practicing the invention consists in exposingthe surface of the metal powder particles to the action of aluminumchloride, in a stream of hydrogen at a temperature between 900 and about1400 C. which is sufficient to decompose the aluminum chloride compoundand to deposit aluminum atoms on the metal particle surfaces and causethese atoms to diffuse into the interior of the particles and to alloywith the metal of the particles. It is believed that the resultingreactions take place either according to equation,

AlC1s+Hz- Al+I-IC1 (1) or according to the equation,

AlC1 +meta1 Al+metal chloride (2) or simultaneously according to boththese equations.

The HCl, which is formed according to Equation 1 as well as the metalchloride, which is formed according to Equation 2 and which is gaseousat the temperature of reaction, is carried away by the stream ofhydrogen, and the aluminum atoms are deposited on the surfaces of themetal powder particles, and will alloy with the surface layer of themetal particles.

The aluminum chloride may be evaporated and mixed with hydrogen outsidethe reaction chamher, but it may also be produced in the reactionchamber, e. g. by reaction of hydrochloric acid with either aluminum orferroaluminum. In a suitable procedure, which will be described in moredetail below, the powder body which is to be surface-alloyed is packedin a mixture of ferroaluminum and an inert ceramic material, and agaseous mixture of HCl and hydrogen is passed through the pack toproduce first A1C13 by reaction of H01 with ferroaluminum and then causethe A1013 to decompose at the surface of the material to besurface-alloyed. When such pack containing ferroaluminum is used, thereaction temperature must be kept well below the melting point of theferroaluminum. In the case of ferroaluminum containing 50% of aluminum,e. g. it is good practice to carry on the treatment at a temperature nothigher than 950 C.

In accordance with one phase of the invention, the soft iron particlesare first sintered into sinter .cake pieces within a reducingatmosphere, such as pure dry hydrogen, or purified dry cracked ammonia,at an elevated temperature so as to produce a porous sinter cake of lowdensity, not higher than about 4 to 5 g./cc., and of low strength nothigher than modulus of rupture of about 6500 to 7000 p. s. i. The powderparticles of such porous low density sinter cake pieces prointerior ofthe powder particles.

duced by the initial sintering treatment, may be readily surface-alloyedwith aluminum by known processes for depositing aluminum from analuminum compound at an elevated temperature at which the depositedmetal difiuses into the The porous lowdensit sinter cake pieces do notundergo any material increase in density and strength when submitted tothis surface-alloying treatment, and they may be readily comminuted withminimized work hardness into minute soft powder particles of therequired size range and high degree of plastic deformability which makesit possible to compact the resulting surface-alloyed ferrous powderparticles with a pressure of only 50,000

. p. s. ipinto strong green metal powder compacts having a modulus ofrupture of 400 p. s. i. or more, without adding any binder to the powderparticles.

In accordance with another phase of the invention, powder particles ofthe oxides of ferrous metals, such as iron, nickel and cobalt, ormixtures of such oxides, or mixtures of metals and oxideswith or Withoutadditional mixtures of powders of oxides or metals of the groupconsisting of molybdenum, tungsten, chromium, vanadium and manganese, orpowders of these metals, or mixtures of powders of these metals andtheir oxidesare initially sintered into porous sinter cake pieces at anelevated temperature within a reducing atmosphere so as to producesinter cake pieces of the reduced metal powder particles having a lowdensity not higher than about 4 /2 to 5 g./cc., and of low strength nothigher than a modulus of rupture of about 7000 p. s. i. The metal powdersinter cake pieces obtained by such initial reducing treatment are thensurface-alloyed with aluminum by any known process for depositingaluminum from an aluminum compound at an elevated temperature at whichthe deposited aluminum difiuses into the interior of the metal powderparticles. The low-density sinter cake pieces subjected to suchsurface-alloying treatment do not undergo any material increase indensity or strength, and

may be readily comminuted into minute powder particles of the requiredsize range which have only a minimized work hardness, and exhibit a highdegree of softness and deformable plasticity which makes it possible tocompact the resulting aluminum-surface-alloyed minute metal powderparticles with a pressure of only 50,000 p. s. i. into strong greenpowder compacts having a modulus of rupture of 400 p. s. i or more,without adding any binder to the powder particles. The surface-alloyedmetal powders produced in the manner described above from oxides of thedifierent metals will yield a powder body, the individual particles ofwhich are alloys of the different metals and substances corresponding tothe different metal oxides used for making the initial sinter cakepieces and the proportionof aluminum deposited on the sinter cakeparticles.

A convenient way for producing such sinter cake pieces for the foregoingtreatment is to mix the powders of the metal oxides with a lubricant andbinder and pellet the mixture into pellets, the pellets containingenough of the organic lubricant and binder which decomposes and isdriven off at the sintering temperature so that the resulting sintercake pellets or pieces have the required high porosity and the requiredlow density and strength.

Without in any way limiting the scope of the invention, there Will behereinafter described exsimples ofsati'sfaetcry procedures for producingsoft aluminum-surfacealloyed' metal. powders of the invention fromcommerciallyavailable raw material.-

There willnowbe described by wayof example, a sat'i'sf-actoryprocedurefor producing aluminum-surface-alloyed corrosion-resistant soft ironpowder." of the inventionfrom commercially available"-sp'onge: ironpowder having a carbon oontent-of aboutalfi A typical-commercialiapowder of this type used in the process contained: amix-ture of minute powder particleshaving following particlesizedistribution typical ofium'oldable metal powders. 21% 100, +160: mesh,30%v -150,. +200 mesh; 13%- -200, +250 mesh, 14%. 250, +325 mesh, and'the balance. -325 meshvpowder.

The iron powder is subjected to. an initial treatment. which sinters: itinto a porous iron powder "cake-of low density and high porosity so asto: permitready surface-alloying of the cake powder particleswitlnsilic'on. deposited. thereon. Such porous low-density sinter cakesmay be. produced as follows: A layer of the iron powder about /8 to /4"thick. is deposited into. sinter boatsof suitablev heat. resistant metalsuch as iron-chromium. sheet metal,.-without in any way compactingthedeposited iron powder. The interior surf-ace or. each sinter boat iscoated with a*--stick -suppnessing or. release medium such a a water'suspension of magnesium hydroxide to permit ready" separation of thesintered powder body from the boat after-the sintering operation. The.powder within the boat is then treated within. afurnace'in a. reducingatmosphere, such as purified. dry hydrogen or purified cracked ammonia,atra temperature of about 800 to 1050 C. for "one-half to three hours,and then permitted to cool within thesame atmosphere to roomtemperature. In general, such treatment at a temperature'betweenabout900 to 1000' C., such as 950 C-.,tf-or one hour, is sufficient forsintering commercial sponge. iron. powder into a sinter cake having adensity-of .1.57 to A g./cc., which is readily frangible-into lumps orsinter cake pieces suitable for undergoing: the desired surface-alloyingtreatment.

A good way for carrying onthe foregoing sin.- tering treatment-is topush the powder containing-boats through a tunnel "furnace within whichis'maintained the desired reducing atmosphere, the "heat treatment beingfollowed by cooling within the same atmosphere as a part of a continuousprocess, in the course of which the boats are moved from oneend of thefurnace to the other.

After completion of the initial sintering treatment, the iron powdersinter cakes are removed from the boats and are broken into sinter cakefragments or pieces of asize suitable for the subsequent aluminizingtreatment. sinter cake pieces having cross-sectional dimensions in therange between about inch to about 1% or inch have beenfound suitable forthe subsequent surface-alloying. treatment.

example of. a satisfactory procedure for surface-alloying. such ironpowder sinter cake pieces with aluminum. will now be described: Theporousiron powder sinter cake pieces are packed withinia packmass which:is eifective in depositing aluminum on the sinter cake powderparticles. The pack may consist, for instance, of 50% by volume ceramiclumps, such as porcelain pieces, the balance an aluminum alloy such asferroa-luminum. containing. (by weightl about? to60% aluminum.

The sinter cake pieces packed; withxporcelaim and ferroaluminum areplaced within treatmentbaskets of suitable heat-resistant metal,.'suclras a chromium-nickel-iron alloy, and; the packed treatment baskets whichare placed within' arretort. The contents of the retortrarewheatedtm an.elevated temperature in the range :be'tweem about 900 and 1100 Cbutwell'belowthe melti-ng temperature of the packing materiaLandianatmosphere containing pure. dry hydrogen: -and v dry hydrogen chloridegas, is passed. through the retort'for producing reactionscaus'ingsal'umiimm; atoms to be deposited on themetalpowder ticles ofthe sinter cake lumps, and to difiusedntm the interior of the metalparticles and alloy therewith. In such surface-alloying treatment" thehydrogen chloride gas passing through. the; retort interacts with theferroaluminum; to; form: aluminumchloride gas. The AlCla gasisdecomiposed and the aluminum atoms depositedon the. surfaces of thepowder particles. diffuse into-the interior of the particles and alloywiththe: metal: of the particles.

By way of example, the following; surfaces alloying treatment gavesatisfactory resultsi. The: broken up sinter cake lumps arepacked inacircular treatment basket havingga. diameter of 22" and a height of10". As packing 'materialvthere was. used a mixture of equal parts ofporcelain and ferroaluminum lumps, the ferroalumi-num containing equalproportionsv of aluminum and iron. Four packed baskets are stacked uponeach other in a closely fitting retort of 60" height. .A mixture of 3parts of gaseou HCl and 20 parts of E2 was passed, at 940 0., throughthe retort, first for 2 hours at a rate of approximately 40 cu. ft. perhour, and then for 10 additional hours at a rate of approximately 20 cu.ft. per hour. After being subjected to the foregoing surfacealloyingtreatment, the resulting sinter cakes have about the same density andthe same strength or modulus of rupture: which they exhibited prior tothe surface-alloying treatment. Because of the low density and strengthof the surface-alloyed sinter cake pieces, theymay be readily comminutedwith minimized work hardness into soft metal powder particles of. a.size range and high degree of plastic deformability required forcompacting the powder particles into strong pellets with relatively lowcompacting pressure of only 50,000 p. s. i. into green com.- pactshaving a modulus of rupture of at least 200 and higher.

By way of example, surface-alloyed iron powder sinter cakes prepared inthe manner described above, were disintegrated by a disc crusher intosurface-alloyed powder particles, of which passed through a mesh screen,the powder containing about 8% 325 mesh particles, about 50% 100, +200mesh particles, balance 200, +325 mesh particles. Green tests of suchsurface-alloyed powder particles compactedwithout any binderunderapr'essure of 25 t. s. 1. exhibited an average modulusof rupture ofabout 850 p. s. i. (pounds per square inch);

The surface-alloyed iron powder particles :of the invention, produced bythe. treatment of, the invention, have a spongy character, tentacle-likeshape. Green compacted bodies produced .from such powders exhibitexcellent corrosion-resistance at elevated temperatures.

Similar results are obtained with other types of soft, plasticallydeformable iron powders subjected to the aluminum-surface-alloyingtreatment of the invention. In fact, as explained above, many relativelyhard and difficult to compact iron powders will, in general, whensubjected to the aluminum-surface-alloying treatment of the invention ofthe type herein described, become. softened and acquire the high degreeof plastic deformability which renders them readily moldable into stronggreen compacts of high strength of at least 200 p. s. i.

The foregoing rupture test data were obtained by test equipment and testmethods described in the article of J. P. Scanlan and R. P. Seelig inPowder Metallurgy Bulletin, 44, p. 128 (1949), using test bars 1 /2"long, {'g" wide, and /4" thick.

The test results given above are representative of. the characteristicdistinguishing aluminum surface-alloyed ferrous powders of the inventionover prior art of corresponding compositions. Thus, softaluminum-surface-alloyed and corrosion-resistant ferrous powders of theinvention difier from heretofore available corrosion-resistant ferrouspowders by the fact that green com pacts made from powders of theinvention have a materially greater rupture strength, at least two tofour times greater than prior art powders of similar composition.

A distinguishing characteristic of the corrosion-resistantsurface-alloyed soft ferrous powders of the invention is the fact thatwhen compacted into a green test bar body of the dimensions given aboveunder pressure of 25 t. s. i. with no lubricant or binder, suchcompacted green body exhibits a modulus of rupture several times greaterthan a similar body produced by compacting corresponding prior artpowders, under the same pressure. In particular, such green test barcompacts made from powders of the invention have a modulus of rupture ofat least 400 p. s. i.

In producing surface alloyed soft, plastically readily deformableferrous powders of the invention, it is important that at all stages ofthe processing operations, the powder particles should not be subjectedto any material work hardening forces. Thus, for instance, it isessential that in initially sintering the soft iron powder particlesinto porous sinter cake fragments suitable for packing into the alloyingpack, the powder which is to be subjected to the initial sinteringaction should not be compacted under any substantial pressure. If asubstantial compacting pressure, even as low as t. s. i. is initiallyapplied to such soft ferrous metal powder '(Fe, Ni, Co) in preparationfor the initial sintering process, which precedes the aluminumalloyingtreatment, such surface-alloyed sinter fragments will acquire arelatively great density and strength, and large forces will be requiredfor crushing them, and the resulting powder particles will be severelywork hardened. Unless the sinter cake fragments which are to besubjected to the surface-alloying treatment are of low density andstrength-obtainable if the powder particles subjected to the preliminarysintering action have not been initially compacted under pressure-thecrushing energy required for pulverizing the alloyed sinter cakefragments would be so large that the resulting surface-alloyed powderparticles would be distorted in shape and work hardened, making itnecessary to apply undesirably large pressure for compacting them intobodies of the required final shape.

In other words, when producing aluminumsurface-alloyed soft ferrouspowders of the invention, it is essential that the sintered powder cakefragments or lumps-produced in preparation for the surface-alloyingtreatment-should have a low density, and correspondingly low strength,so as to minimize any work hardening imparted to the powder particleswhen pulverizing the sintered powder cake fragments or lumps followingthe surface-alloying treatment, thus resulting in powder particlesexhibiting only minimized work hardening. Furthermore, it is alsoessential that the sintered powder cake fragments or lumpsproduced inpreparation for the surface-alloying treatment-should have high porosityso that they are permeable to the gaseous metal compound by means ofwhich thesur face-alloying treatment is carried on.

By controlling the density and strength of the sinter cakelumps-produced in preparation for the surface-alloying treatmentit ispossible to control the preparation of soft, plastically deformablealuminum surface-alloyed powders of the invention.

As explained above, the proper range of the density of the sinter cakelumpsproduced in preparation for the surface-alloying treatmentis about1.5 to 4 g./cc. The modulus of rupture of such sinter cake lumps iscorrelated to their density, being about 50 p. s. i. for sinter cakelumps having a density of 1.7 g./cc. and increasing to about 6500 p. s.i. for sinter cake lumps having a density of 4 g./cc.

Furthermore, as long as the sinter cake lumps, produced in preparationfor the surface-alloying treatment are of low density and have a lowmodulus of rupture after being subjected to the surface-alloyingtreatment, aluminum is surfacealloyed with the individual powderparticles of such sinter cakes.

Accordingly, to obtain aluminum-surfacealloyed powders of the invention,it is sufficient to control the density and/or modulus of rupture of thesinter cake lumps produced in preparation for the surface-alloyingtreatment. As long as the density of such sinter cake lumps is not morethan about 4 g./cc., and their modulus of rupture is not more than about6500 p. s. i. they will, after the surface-alloying treatment, remain ofsufficiently small density andstrength as to permit their pulverizationwithout material work hardening of the powder particles. As a result,the surface-alloyed powder obtained from such sinter cakes will have thedesired high degree of softness and plastic deformability as to make itpossible to compact such powders into green compacts having a modulus ofrupture of at least 400 p. s. i. and higher.

By sintering such green compacted bodies of aluminum-surface-alloyedsoft iron powder of the invention, compacted with only a relativelysmall pressure, there are obtained bodies having the same strength asthose produced by prior art powders of similar composition which havebeen compacted with much higher pressure. Good results are obtained bysintering green compacted bodies of such aluminum-alloyed powder at atemperature in the range of l200 to 1350 C. within a protectiveatmosphere such as dry hydrogen, or of purified cracked ammonia. It isalso desirable to maintain the protective atmosphere at a dew point ofabout .50 C. or below. Such 10w dew point atmosphere may be obtained bya suitable getter such as pure chromium powder or ferro-silicon powder(70% Gr),

mixed with aluminum oxide.

Green compacted bodies made from soft surface-alloyed iron powder of theinvention may be infiltrated with other metals such as copper and copperalloys for producing composite bodies having the desired combinedcharacteristics. Such infiltrated bodies may be subjected to heattreatments, eitherat the time of the infiltration, or subsequent theretofor causing the infiltrant and the metal particles and substances of thepowder compact skeleton to diffuse into each other to any degree asdesired.

According to a further phase of the invention, aluminum-surface alloyedsoft plastically readily deformable powders of either iron, nickel orcobalt, or alloys or mixtures thereof, are produced in a very economicalway from the oxides, or mixtures of the oxides of these metals, ormixtures of these oxides and metals. Among the oxides suitable for' thispurpose is black mill-scale containing principally iron oxides- (F6304and FeO') which is formed when rolling and forging iron: and steel.

For producing aluminum-surface-alloyed' powders of the invention fromoxides of iron, nickel -or cobalt, or' mixtures of the different oxides,

the desired metal oxide or oxides may be subjected to a combinedreducing and sintering treatment.

By way of example, there will now be described a satisfactory processfor producing aluminum-alloyed. powder containing soft moldable iron andnickel alloy powder particles partially aluminum-alloyed in accordancewith theprocess of the invention. Powder particles of iron oxidemill-scale containing: essentially F8203 and FeO are mixed with nickeloxide powder and lamp black, andball milled into a powder' of -100 mesh.As an example, 200 parts of'the mill scale are mixed with 226 partsnickel oxide and one part of: lamp black. The powder mixture is thenball milled to powder of 100 mesh, and placed in treatment boats ofheat-resistant metal coated on the interior with a stick-suppressingmedium, and treated ina. furnace under reducing. atmosphere such as dryhydrogen; or cracked ammonia at a. temperature inv the range between800- and 11100:? C.. for one-half to three hours; and" then permitted tocoo11in;tl"ie. same atmosphere: Satisfactory results are obtained: bysuchv treatment carriedon at: a temperature of about: 950 C, for onehour. The. powder mix'may bepelleted in.- to pellets before subjectingit. ill)? the foregoing treatment, or it maybe: placeddnto theitreatmentboat as a thin powder layer about A;.- to A deep, in-whichcasethe:resulting-sinten'cake produced by? the reducing heat: treatment isbroken up intolumps. The powder is. formed into pellets by mixing: it?with a: lubricant and binder so that'the resultantimixed powderrmass maybe readily made upinto smalLporousapelilets, for instance, ofcylindricalishape;havingza diameter of about A; to inch, and th'e sameheight, with a pellet-density of '3ito' 4 grams per cubic centimeter-1Any suitable organic lubricant and binder which decomposes at elevatedtern:- peratures of about 800 to' 900 C. and above, may be used asalubricant and binder inlmak ing such pellets; For'instance, high'fatty'a'cids, such as stearic acid, and salts of'st'earic acid, such as zincstearateand=the2 like; are suitable for'use asa= lubricant.Carbohydrates; suchias dextrose dissolved in water; or" camphor' disi-10 solved in alcohol, may be used as a binder in making such pellets.

The reducing and sintering treatment carried out at an elevatedtemperature in the manner described above reduces the oxides and causesthe different metal constituents of the different powder particles, i.e. of nickel and iron powder to mutually dii fuse with each other, andthe individual powder particles become actually alloyed.

In the particular example referred to herein, the reducing and sinteringtreatment was performed at a temperature of about 1050" C. for one hour,resulting in sinter cake pellets having a density of about 2.3 g./cc'.,with a weight loss of about .5%.

The reduced sintered powder pellets or sinter cake pieces produced inthe manner described above are then subjected to analuminum-surface-alloying treatment similar to that applied to sintercake pieces made from spong'e iron powder as described above. Thesurface-alloyed sinter cake pellets obtained by such treatment haveabout the same density as they had before i being subjected to thesurface-alloying treatment, and they may be comminuted with minimizedwork hardness into s'oft surface-alloyed powder particles having thedesired high degree" of plastic deformability. Byway of examplesurface-alloyed sinter cakepellets produced' in the manner describedabove, yield upon comminution, a surface-alloyed iron powder of highsoftness, and plastic deformability, so that a test bar of such powdercompacted under a pressure of 25 t. s. i without any lubricant orbinder, exhibits amodulus of rupture of about 900 p. s. i.

By prolonging the' surf a'ce-alloying treatment, or by subjectingthesurfa'ce-alloyed sinter cake pieces, such as the sinter'cakefragm'entsor sinter cake pellets, to a suitably long additional heat orsi'nt'ering' treatment, the aluminum content of the" o'uter'layers' ofthe powder particles of such bodies may be caused to diffuse into theinterior of the individual powder particles, and thus cause suchpowderparticles to be substantially uniformly alloyed. Furthermore,Suchprolonged diffusion treatment will cause the: difie'rent metals ofthe individual powder particles, as well as their aluminum content, tomutually diftu'se, thereby giving the resulting body. the characteristicdesired alloy composition.

By proceeding. in a similar manner with a mixture of 'oxides of one'ormore of the metals iron, nickel andc'obalt, with one or more ofthemetals molybdenum, tungsten, chormium, vanadiuni, manganese;or'the'o'xides of these metals, there may beproducedaluminum-surface-alloyed soft, plastica-lly readily" deformablepowder mixtures-of the desired different metals, to wit, iron, nickel,cobalt, molybdenum, tungsten, chromium, vanadium, and manganese, exhibii'ng th'e' combined characteristics of the different combinedconstituents;

It should be noted" that when producing surface-alloyedsoft metalpowders of the invention from oxides of the desired metals, such asoxides of iron, nickel, cobalt, molybdenumand tungsten, and mixturesthereof, the oxidepowders may be pelleted" into pellets for thepreliminary reducing and sintering treatment; When such pellets aresubjected to the combined reducing and sintering treatment of the typedescribed above, the admixed'lubricant and binder is decomposed" anddriven off. As a result, the reduced sinter cake pellets have therequired low density and small strength, comparable to the strength ofthe sinter cake fragments produced by sintering sponge iron powderdeposited in a layer of about /4" within the treatment boats in thepreliminary treatment of sponge iron powder described hereinbefore. Aslong as the sinter cake pellets resulting from the preliminary reducingand sintering treatment have a density not exceeding about 5 g./cc.. anda modulus of rupture not exceeding about 6500 p. s. i. the sinter cakepellets will yield aluminum-surface-alloyed powders having the desiredhigh degree of softness and plastic deformability as to make it possibleto compact such powders into green compact bodies having a modulus ofrupture of at least 400 p. s. i. and higher with a pressure of only50,000 p. s. i.

As used in the specification and claims, the expression sinter cakelumps includes both sinter cake lumps and sinter cake pellets ofsufficiently low density and strength that upon comminution of suchsilicon-alloyed sinter cake body into minute silicon-alloyed powderparticles having the desired high degree of softness and plasticdeformability which makes it possible to compact such powders into greencompacted bodies having a modulus of rupture of at least 400 p. s. i.and higher with a pressure of only 50,000 p. s. i.

Our copending application, Ser. No. 153.286 filed March 31, 1950,discloses and claims novel, readily compactible and moldable soft metalpowders of loose powder particles composed of at least one metal of thegroup consisting of iron. nickel, and cobalt, with or without at leastone metal of the group consisting of molybdenum and tungsten, and ofmixtures and alloys of said metals, the surface layer of the particleshaving alloyed therewith chromium deposited on the surfaces of theparticles from a chromium compound at an elevated temperature so as togive them effective resistance to oxidation, the particles havingsufficiently great softness so that when compacted in the absence of abinder into a green body under a pressure of about 50,000 pounds persquare inch, the resulting green body will exhibit a modulus of ruptureof at least about 200 pounds per square inchand also novel methods ofproducing such oxidation-resistant soft metal powder.

Our co-pending application, Ser. No. 153,288, filed March 31, 1950,discloses and claims novel, readily compactible and moldable soft metalpowders of loose powder particles composed of at least one metal of thegroup consisting of iron, nickel, and cobalt, with or without at leastone metal of the group consisting of molybdenum and tungsten. and ofmixtures and alloys of said metals, the surface layer of the particleshaving alloyed therewith silicon deposited on the surfaces of theparticles from a silicon compound at an elevated temperature, so as togive them effective resistance to oxidation, the particles havingsufliciently great softness so that when compacted in the absence of abinder into a green body under a pressure of about 50,000 pounds persquare inch, the resulting green body will exhibit a modulus of ruptureof at least about 200 pounds per square inchand also novel methods ofproducing such oxidation-resistant soft metal powder.

According to another phase of the in ention, soft. readily compactiblemoldable metal powders of loose powder particles composed of at leastone metal of the group consisting of iron, nickel, and cobalt, with orwithout at least one metal of the group consisting of molybdenum andtungsten, and of mixtures and alloys of said metals, are surface-alloyedwith at least two substances of the group consisting of chromium,silicon, aluminum, by depositing on the surfaces of the metal powderparticles the different alloying substances from compounds of thedifferent alloying substances at an elevated temperature so as to causethe deposited atoms of the alloying substances to difiuSe into at leastthe surface layers of the metal powder particles.

Such soft metal powders of the invention having surface-alloyed with thepowder particles at least two of the substances of the group consistingof chromium, silicon, aluminum, may be produced by subjecting the powderparticles in sequence to the different surface-alloying proceduresdisclosed in our said applications and the present application, forinstance, by first subjecting the metal powder particles to asurfacealloying action with chromium deposited thereon, followed bysubjecting the metal powder particles to a surface-alloying action withsilicon deposited thereon, followed by subjecting the metal powderparticles to surface-alloying action with aluminum deposited thereon, orby proceeding in a different sequence with such successive differentsurface-alloying treatments.

However, the sinter cake pieces may also be subjected to simultaneoussurface-alloying action of several or all the desired surface-alloyingsubstances. Thus, by way of example, the following procedure may be usedfor producing oxidationresistant metal powders of the invention bysubjecting sinter cake pieces formed of the soft metal powder particlesto the surface-alloying action with chromium, silicon and aluminumdeposited from compounds of these substances. sintered cake pieces ofthe metal powder which is to be surface-alloyed in accordance with theinvention is packed in a pack mass consisting of equal parts offerrochromium, ferrosilicon, ferroaluminum, and a corresponding volumeof porcelain pieces. A gaseous stream of hydrogen and H01 is then passedthrough a retort within which the sopacked sinter cake pieces have beenplaced to cause chromium, silicon and aluminum of the compounds of thesubstances produced within the retort to be deposited on the powderparticles of the sinter cake pieces in the same manner as described inour applications referred to hereinabove until the powder particles aresurface-alloyed with the desired content of the surfacealloyingsubstances. The balance of the procedure is generally similar to theother surfacealloying procedures described in our aforesaidapplications.

The principles of the invention described above in connection withspecific exemplifications thereof, will suggest various othermodifications and applications of the same. It is accordingly desiredthat the present invention shall not be limited to the specificexemplifications shown or described therein.

We claim:

1. A sintered cake body of metal powder particles which is readilycomminutable into soft metal powder particles, the particles of saidcake body being composed essentially of at least one metal of the groupconsisting of iron containing at most about 2% carbon, of nickel, and ofcobalt, and mixtures and alloys of said metals, said cake body havingbeen subjected to an aluminum-alloying treatment causing at least thesurface layer of said particles to become alloyed with aduminum:depositedi. on: the particles from: an

aluminum. compound at elevated temperatnresl'in which; at: least."- 3%?of the. :metal Off; said? b'ody-i'is -displacedibyaluminum, saidcake:.body having at most a density of about 5 gramsrperr-cubiccentimeter; whereby. SBiCl-ibOdYf may-be pulverized into: minutealuminumsalloyed 1 powder: particles whichexhibit:minlmized'iworlr"hardness: and :suf'; fi'ciently' greatsoftnesss'.='.tha:ti.wh'enr;thee'alumn num a'lloyedi-particlesiarercompacteddntoa=green body: unders-pressure of about 150,000." poundsper facezlayer.of saidi'particlesztobecome: alloyedzwith aluminum: deposited. on. theparticles, froman aluminum". compound-- at elevated w temperatures;said: cake body containing at: least .3 aluminum and having at most: adensity of: aboutgrams per cubiccentimeter:

3: .A sinteredcakerbodyi of: metal powder particleszwhichiswreadilyg'comminutable-- into softmetallpowderparticles,;.theiparticles of saidcake bodybeingcomposed-ressentially: of: at-1east one metal of: thegroup-consisting of= iron-containing at-rmost 'ab.out":.2.%.' carbon, ofnickel, and of. cobaltytogether'with" atzleast :onecmetaliof :the 1group consisting of molybdenum and tungsten; andof mixturep and; alloysof said metals; said cake body; having; been subjected to analuminumalloyingtreatment causing at least the surface layer. of. saidparticles. to. become alloyed with aluminum deposited on the particlesfrom a gaseous aluminumcompound-at elevated temperatures, said cakebody-having at most a density of: about 5 grams'pen-cubic; centimeter.and a modulus ofirupturetoflat.most-6500 pounds per square inch, wherebysaidbody may be pulveriz'ed into minute a'luminum alloyed powderparticles whichexhibit minimized work hardness and suinciently great"softness so that whom the aluminum-alloyed particles are compacted intoa green body under pressure of about 50,000 pounds per square inch, saidgreen body exhibits a modulus of rupture of at least 200 pounds persquare inch.

4. A sintered cake body of metal powder particles which is readilycomminutable into soft metal powder particles, the particles of saidcake body being composed essentially of at least one metal of the groupconsisting of iron containing at most about .2% carbon, of nickel, andof cobalt, together with at least one metal of the group consisting ofmolybdenum and tungsten, and of mixtures and alloys of said metals; saidcake body having been subjected to an aluminumalloying treatment causingat least the surface layer of said particles to become alloyed withaluminum deposited on the particles from an aluminum compound atelevated temperatures, said cake body containing at least 3% aluminumand having at most a density of about 5 grams per cubic centimeter.

5. The method of providing a readily compactible and moldable metalpowder comprising first producing readily frangible porous sinter cakepieces composed essentially of sintered metal powder" particles: of atleast one metali off. the group consisting ofironiconta-iningratimost:about .2% carbon, of-x'nickel; and ofcobaltyandrmixatures and: alloys. of: at? least two; of: said; metalssozthat: said" cake.=pieces haveiat Lmost: aidensity of l'abouti4 grams-.per' cubiczcentimeter; anditherer' after:subjecting:saidicake:piecesto: an aluminium, alloying: treatment in .which; aluminumafrom: analuminum .compoundeiss deposit'edwzoni: thezpowder particlesiofsaidz'cake pieces; and icausedzto-diffinse into: said; powder."particles so that. said. powder particles contain: at: least about: 3%:aluminum; andithereaftencomminuting:saidicakezpieces-sinto minute*pl-astically readilyrdeformablemetalpow;- der particles having, the?deposited :aluminum; a'l loyedi therewith: sa': that when. saidalum-mums alloyed particles are; compacted in'. the absence offa -binderinto agreentbody under; aipressureof about: 50,000. pounds-apers squaresinch;said green body exhibits; a; modulus. ofv rupture; of: at: leastabout:20'0:-pounds*per'squareinch".

6: The: method" of providing: as readily? come pa'ctib'le: and":moldable: metal -.powder comprising first: producing? readily frangible;porous: sinter cake P160655: of; sintered; metal powder: particlescomposed essentially of E at. least gonee metaliiofl the group:consisting =of:iron containing ainmostsabo'ut .2%. carbon, of: nickel;.and: of: cobalt; together witli'aup' to atuleastr one.:-metal of;thegroupzcona sistingioffmolybdenumrand:tungsten; andtofrmixe turesand'ialloys of saidametals'g: sothatzsinter cake pieces-have atamostj a:density: of; about- 4 grams perscubic centimeters. and: thereafter :snb,jectin'g'rsaidicake pieces; 1203311: aluminumealloying treatment inwhichzaluminumfromnnaluminum compound is depositedi on; the: powder;-particles of'isa'idcake pieceszand :caused :to: diffuseintoesaidpowderparticlesso zth'attthe said-:powdenparticles contain'atueastabout's3l% aluminum; andithere: after'comminuting saidtcak'epiecessintmminute plastically; readilyi-deformabdemeta-hpowdenparticles: having: the deposited aluminum alloyed therewithtso that; when. sanl a'luminumealloyed particle'srare;compacteddni-the;absencezof a binder into a'sgreenr. body: underya pressure: of: about50;0005: pcundsmcnsquare: inch, send green body exhibitsaas. modulus of.ruptureoffat; leasti about 200Ipoundsper.squareinch:

7. The methodrofiprovidingazreadilwcompact? ible and moldable metalpowder comprising first producing readily frangible porous sinter cakepieces composed essentially of sintered metal powder particles of thegroup consisting of iron having at most about .2% carbon, of nickel, andof cobalt, and mixtures and alloys of at least two of said metals sothat said cake pieces have at most a density of about 4 grams per cubiccentimeter, and thereafter subjecting said cake pieces to analuminum-alloying treatment in which aluminum from a gaseous aluminumcompound is deposited on the powder particles of said cake pieces andcaused to diffuse into said powder particles so that said powderparticles contain at least about 3% aluminum, and thereafter comminutingsaid cake pieces into minute plastically readily deformable metal powderparticles having the deposited aluminum alloyed therewith so that whensaid aluminum-alloyed particles are compacted in the absence of a binderinto a, green body under a pressure of about 50,000 pounds per squareinch, said green body exhibits a modulus of rupture of at least 200pounds per square inch.

8. The method of providing a readily compactible and moldable metalpowder comprising first producing readily frangible porous sinter cakepieces composed essentially of sintered metal powder particles of atleast one metal of the group consisting of iron containing at most about.2% carbon, of nickel, and of cobalt, together with up to about 30% ofat least one metal of the group consisting of molybdenum and tungsten,and of mixtures and alloys of saidmetals so that said sinter cake pieceshave at most a density of about 4 grams per cubic centimeter, andtheresaid green body exhibits a modulus of rupture of at least about 200pounds per square inch.

9. The method of providing a readily compactible and moldable metalpowder comprising first preparing a powder mass of powder particlescomposed essentially of at least one metal of the group consisting ofiron containing at most about .2% carbon, of nickel, and of cobalt, andmixtures and alloys of at least two of said metals, at least some ofsaid powder particles containing the metal in the form of an oxide,heating the powder mass in a reducing atmosphere at a temperature of atleast about 700 C. to produce porous sinter cakes having at most adensity of about five grams per cubic centimeter, and thereaftersubjectin said cake pieces to an aluminumalloying treatment in whichaluminum from a gaseous aluminum compound is deposited on the powderparticles of said cake pieces and caused to diffuse into said powderparticles so that the said powder particles contain at least about 3% ofaluminum, and thereafter comminuting said cake pieces into minuteplastically readily deformable powder particles of said metals havingthe deposited aluminum alloyed therewith so that when saidaluminum-alloyed metal particles are compacted in the absence of abinder into a green body having a pressure of about 50,000 pounds persquare inch, said green body exhibits a modulus of rupture of at leastabout 200 pounds per square inch.

10. The method of providing a readily compactible and moldable metalpowder comprising first preparing a powder mass of powder particlescomposed essentially of at least one metal of the group consisting ofiron containing at most about .2% carbon, of nickel, and of cobalt,together with up to about 30% of at least one metal of the groupconsisting of molybdenum and tungsten, and of mixtures and of alloys ofsaid metals; at least some of said powder particles containing the metalin the form of an oxide, heating the powder mass in a reducingatmosphere at a temperature of at least about 700 C. to produce poroussinter cakes having at most a density .01 about five grams per cubiccentimeter, and thereafter subjecting said cake pieces to analuminumalloying treatment in which aluminum from a gaseous aluminumcompound is deposited on the powder particles of said cake pieces andcaused to diffuse into said powder particles so that the said powderparticles contain at least about 3% of aluminum, and thereaftercomminuting said cake pieces into minute plastically readily deformablepowder particles of said metals having the deposited aluminum alloyedtherewith so that when said aluminum-alloyed metal particles arecompacted in the absence of a binder into a green body under a pressureof about 50,000 pounds per square inch, said green body exhibits amodulus of rupture of at least about 200 pounds per square inch.

GEORGE STERN. S. J. SINDEBAND. J. P. SCANLAN.

References Cited in the file of this patent FOREIGN PATENTS NumberCountry Date 598,181 Great Britain Feb. 12, 1948 OTHER REFERENCESTreatise on Powder Metallurgy, vol. I, pages 71 and 74. Edited byGoetzel. Published in 1949 by Interscience Publishers, Inc., New York.

1. A SINTERED CAKE BODY OF METAL POWDER PARTICLES WHICH IS READILYCOMMINUTABLE INTO SOFT METAL POWDER PARTICLES, THE PARTICLES OF SAIDCAKE BODY BEING COMPOSED ESSENTIALLY OF AT LEAST ONE METAL OF THE GROUPCONSISTING OF IRON CONTAINING AT MOST ABOUT .2% CARBON, OF NICKLE, ANDOF COBALT, AND MIXTURES AND ALLOYS OF SAID METALS, SAID CAKE BODY HAVINGBEEN SUBJECTED TO AN ALUMINUM-ALLOYING TREATMENT CAUSING AT LEAST THESURFACE LAYER OF SAID PARTICLES TO BECOME ALLOYED WITH ALUMINUMDEPOSITED ON THE PARTICLES FROM AN ALUMINUM COMPOUND AT ELEVATEDTEMPERATURES IN WHICH AT LEAST 3% OF THE METAL OF SAID BODY IS DISPLACEDBY ALUMINUM-ALLOYED POWDER PARTICLES AT MOST A DENSITY OF ABOUT 5 GRAMSPER CUBIC CENTIMETER, WHEREBY SAID BODY MAY BE PULVERIZED INTO MINUTEALUMINUM-ALLOYED POWDER PARTICLES WHICH EXHIBIT MINIMIZED WORK HARDNESSAND SUFFICIENTLY GREAT SOFTNESS SO THAT WHEN THE ALUMINUM-ALLOYEDPARTICLES ARE COMPACTED INTO A GREEN BODY UNDER PRESSURE OF ABOUT 50,000POUNDS PER SQUARE INCH, SAID GREEN BODY EXHIBIT A MODULUS OF RUPTURE OFAT LEAST 200 POUNDS PER SQUARE INCH